1. Field of the Invention
The present invention relates to a system and an apparatus for matching infrared radiation emitted from aircraft skins to background infrared radiation in order to minimize the contrast between the two radiations, and to thereby camouflage the aircraft from electro-optical detection systems. In particular, the invention relates to the use of heat pipe systems and apparatus for cooling the internal surface of an aircraft skin, and especially the bottom portion of an airframe, for the camouflage of the aircraft from the look-up viewing aspect angles of electro-optical detection systems by transferring heat otherwise emitted by the skin to the fuel which is carried by the aircraft.
Combat aircraft may be readily detected and targeted by passive electro-optical systems which lock-on to the infrared radiation which is emitted from the skins of aircraft. In particular, passive electro-optical systems integrated into missiles, aircraft, and spacecraft are equipped with and employ infrared detection sensors. These systems are adapted to operate from either above or below the target aircraft. In operation, the electro-optical systems take a general reading of the background infrared radiation within the sensor""s field of view. Thus, any voltage changes encountered in the background infrared detection profile may be indicative of the presence of combat aircraft.
Under power flight conditions, especially at low altitudes and high speeds, the temperature of an aircraft skin increases due to aerodynamic heating. Electro-optical systems which are disposed either above or below such combat aircraft may be able to detect the presence of the combat aircraft in response to the difference between the relatively colder earth or sky background infrared radiation and the hotter infrared radiation which is emitted by the aircraft. In general, the aircraft becomes susceptible to detection thereof within the 8 to 14 micrometer wavelength band of the electromagnetic spectrum due to its positive infrared contrast signature when compared with background radiation. Consequently, it is desirable for such combat aircraft to incorporate an infrared control system for matching the infrared signature emitted from an aircraft with the associated terrain and/or sky background associated with the position of the aircraft.
2. Discussion of the Prior Art
A number of possible approaches are available for reducing the infrared radiation emitted by an aircraft. For instance, one approach is to provide ducting enabling the conveyance of cold, ambient air to selected, hot portions of the aircraft skin in order to cool those portions. However, cold air ducting results in an overall increase in the volume or dimensions of the aircraft. Such a dimensional increase has the undesired result of increasing the overall cross-section (projected area) of the aircraft, and thereby renders the aircraft more susceptible to detection by radar, electro-optical, and laser detection systems.
Another approach contemplates the ducting of aircraft fuel to desired sections of the heated skin. However, this approach is also subject to drawbacks in that, once again, the required ducting increases the cross-section of the air frame, with the attendant undesired increased detectability of the aircraft by radar, electro-optical, and laser detection systems. Moreover, the ducting of fuel also increases the vulnerability of the aircraft upon being attacked. For example, any penetration of the ducted area by machine gun fire or projectiles carries with it the potential for igniting a fuel fire due to high temperature exposure. In other words, with a distribution of the fuel throughout the aircraft, there is encountered a considerable increase in the probability of igniting the fuel.
Other approaches have dealt with the utilization of the aircraft fuel as both a heat sink and a heat exchange medium for cooling the aircraft; for example, as disclosed in Mayer U.S. Pat. No. 4,505,124. The system described in that patent necessitates the installation of significant amounts of ducting, evaporators, heat exchangers, and pumps. Consequently, the patented system significantly adds to the overall bulkiness or spatial requirements and to the weight of the aircraft, while increasing the vulnerability of the aircraft to projectile ordinance.
Frosch, et al. U.S. Pat. No. 4,273,304, relates to another aircraft cooling system which employs a separate cooling fluid for removing heat from the airframe and for transferring the removed heat to the aircraft fuel through a heat pump and heat exchanger. This patent also relates to an active system requiring extensive installations of pumps and ducting in the aircraft.
A further aircraft heat exchanger system is described in Sabol U.S. Pat. 3,929,305. This particular system relies upon an impulse tube which is ducted from a coolant reservoir past a heated skin section and to a heat exchanger. Heat from the aircraft skin boils the coolant in the tube so as to produce a bubble which expands and forces the coolant fluid through a closed system. In the heat exchanger, the heat acquired by the cooling fluid is transferred to the aircraft fuel. In this patent, there are also disclosed other types of passive heat exchangers, and in particular, a heat pipe. However, in that disclosure, heat pipes are generally criticized for their dependence upon capillary action, their relatively constricted passages, and their sensitivity to gravity or acceleration forces.
Heat pipes are commonly used in satellites as heat exchangers; for example, as disclosed in Wyatt U.S. Pat. Nos. 3,152,744 and 3,517,730. In the former patent, a heat pipe is referred to for the maintaining of a constant temperature on the solar panels of a satellite; whereas in the latter patent, a controllable heat pipe is disclosed for controllably varying the amount of heat which is removed from predetermined portions of the satellite.
Notwithstanding the various cooling systems and apparatus which are presently known and employed in the technology, there remains an unfulfilled need in the art for the provision of an infrared matching system. In particular, a need is evident for an infrared matching system which eliminates the requirements for ducting, and which is capable of reducing the infrared radiation emitted from the skin of an aircraft.
Accordingly, it is a primary object of the present invention to provide for the novel and unique camouflaging of aircraft, and particularly to protect combat aircraft from detection thereof by infrared detection systems.
A more specific object of the invention is the provision of a substantially uniform temperature throughout the skin of the aircraft in order to camouflage the aircraft against the infrared radiation of the sky and/or terrain background proximate the aircraft, as well as for reducing thermal cyclic stresses acting on the aircraft skin in order to extend the life expectancy of the aircraft skin.
Yet another object of the invention is to transfer heat from the aircraft skin to the fuel of the aircraft in order to preheat the fuel and to thereby increase the overall efficiency of the aircraft engine combustion.
Still another object of the invention resides in the provision for the transferring of heat from the aircraft skin to the fuel contained in the reservoir via the heat pipe.
A further object of the invention relates to the utilization of the heat pipes as structural load carrying members of the aircraft.
The foregoing and other objects of the invention are obtained through the provision of an infrared radiation suppression and matching system and apparatus. The system and apparatus are installed and transported aboard an aircraft and assist in concealing the aircraft from infrared detection systems which may be disposed above, below, or at the same altitude as the aircraft. The portions of the aircraft which generate detectable infrared radiation are known or can be measured through the employment of suitable thermocouples. Heat pipes are attached to applicable interior aircraft surfaces located opposite external heat generating surface portions. Basically, the heat pipe is constructed of an elongated apparatus having a heat receiving end which is connected to the interior surface of the aircraft skin, whereas the other end of the heat pipe is adapted to dissipate heat acquired by the receiving end thereof. The heat dissipating end of the heat pipe is operatively connected, in a heat transferring manner, to the fuel which is stored in a fuel tank carried by the aircraft.
The infrared radiation suppression and matching system includes infrared detectors disposed at various locations about the aircraft for detecting background infrared radiation. In a preferred embodiment of the invention, three such detectors are oriented in three mutually orthogonal directions for monitoring the background radiation emanating and received from such directions. The signals generated by the infrared detectors are transmitted to a suitable control installation, such as the on-board computer of the aircraft. The control installation or computer compares the background radiation signals with the radiation emitted by the aircraft skin.
The power flight conditions (altitude and Mach. number), as determined from the aircraft navigation system, are input into the computer which computes the aircraft skin radiance. The aircraft skin radiance may also be determined (through Planck""s Law) by locating thermocouples at various strategic locations on the aircraft skin. Thermocouples will each convert the airframe thermal energy into an electrical signal which is transmitted to the computer for processing.
When the control installation provides indication of the presence of a significant difference between the background radiation and the emitted aircraft radiation of an extent whereby the aircraft would be detectable by infrared detection systems, a control signal is generated and transmitted to the heat pipe.
In the preferred embodiment, the heat pipe is a controllable element of variable heat dissipating ability. Upon receipt of the control signal from the control installation, the heat pipe will commence transferring heat from the skin of the aircraft to the fuel at a rate proportional to the control signal. The heat pipe continues to operate in effectuating the transference of such heat until the radiation emitted from the skin of the aircraft matches the background radiation.
Heat pipes are considered to be particularly useful in the implementation of the invention in as much as they are passive elements and do not require the installation of extensive ducting. Although heat pipes provide a relatively poor performance only when subjected to heavy acceleration modes, such loads are generally only encountered under conditions of close combat after the aircraft has already been detected and has been acquired as a target by enemy aircraft or enemy ordinance. At such time, infrared radiation matching or suppression is a secondary issue in as much as, in all probability, other means have acquired or detected the aircraft. Hence, any reduced performance by the heat pipe during such combat maneuvers is of little consequence.